1. Field of the Invention
The present invention relates to an image forming apparatus using an intermediate transfer body and in particular, to a technology for improving a transfer performance of an intermediate transfer body.
2. Description of the Related Art
In a color image forming apparatus of an electro-photographic system, there is known a configuration in which by using an intermediate transfer belt, toner images of four colors are superimposed on the intermediate transfer belt and then transferred at once on paper or the like. This is concerned with a technology in which by applying the “superimposition and transfer” which is liable to become unstable from the process standpoint to a stable intermediate transfer belt, the toner images are transferred to the intermediate transfer belt while keeping high image quality and then transferred at once on a final transfer material such as paper. Thus, multiplicity of use of paper can be improved while controlling the degradation of image quality to the minimum. In recent years, in an image forming apparatus employing a so-called “tandem system”, the foregoing intermediate transfer system has become the mainstream.
Also, in recent years, in such an image forming apparatus aiming to realize high image quality, a small-sized toner having an average particle size of not more than 6 μm is being widely used. Following this, in the development processing and transfer processing, realization of higher image quality and higher stabilization is being demanded.
In particular, a transfer step is a step which is frequently accompanied with a discharging phenomenon and which is largely related to the degradation of image quality. For example, in a primary transfer step, the transfer is achieved by the movement of a toner from a photoconductor to an intermediate transfer belt by an electric field formed between the intermediate transfer belt and the photoconductor. Actually, discharging is generated in a region close to and before a contact part between the photoconductor and the intermediate transfer belt, and a part of the toner flies on the intermediate transfer belt.
Since the foregoing discharging phenomenon locally occurs and is generated unstably and continuously, the toner attached onto the intermediate transfer belt is influenced by the discharging from various directions and moves towards a direction of a belt surface, whereby so-called “transfer scattering” is possibly generated. Furthermore, a part of the toner which has moved from the photoconductor to the intermediate transfer belt in a transfer nip section also receives a local force in various directions by the discharging phenomenon and moves in a lateral direction in a region where the photoconductor and the intermediate transfer belt are separated from each other, whereby transfer scattering is generated. The generation of such transfer scattering leads to the degradation of an image in the image forming apparatus.
Also, in a secondary transfer step from the intermediate transfer belt to a body to be transferred such as paper, when discharging is generated before the belt and the body to be transferred come into contact with each, the toner on the intermediate transfer belt moves towards a direction of the belt surface likewise the primary transfer, whereby transfer scattering is generated, too. At that time, since an electrostatic latent image is not present on the intermediate transfer belt, a force to electrostatically fix the position of a toner does not substantially work, whereby the toner becomes in a state that it is liable to move (scatter) towards the direction of the belt surface.
As described above, the image scattering in the transfer step was generated not a little due to the discharging phenomenon generated before and after the transition position in the primary transfer step and secondary transfer step. In addition, the foregoing transfer scattering is more likely influenced in a spherical toner in which an adhesive force of the toner to the intermediate transfer belt becomes easily weak. In particular, this was problematic in the case of using a small-sized spherical toner. Moreover, at the transfer position, when a portion containing a toner and a portion not containing a toner are present, especially when the toner is present in two or more layers, in the case of a toner having a particle size of 5 μm, a thickness difference becomes possibly 8 μm or more. As is clear from the Paschen's law of discharging, the discharging in air is generated from a gap of approximately 6 μm. Accordingly, in a portion of a difference in level by the toner of two or more layers, the discharging is generated in the inside of the transfer nip, too, whereby the degradation of image quality is generated.
Also, since a difference in pressure is also generated in the transfer nip between a region where a toner thickness is present and a region where a toner image is not present, when a line having a thickness to some extent is transferred, “hollow defects” that the center of the subject line turns back to a side of the photoconductor without being transferred are generated, too.